Optical elements have had various uses in many diverse technologies, including sensors, image projectors, displays (e.g., liquid crystal display (LCDs), plasma display, and electro-luminescence display), as well as opto-electronic devices for telecommunications. As the telecommunications industry develops, the need to develop precision optical elements that incorporate microstructures increases. In telecommunication devices, optical elements may be used, for instance, in fiber and laser couplers, optical switches, or as diffraction gratings for WDM applications, and densely packed microlens arrays (MLAs) or networks for wavelength management modules or collimator applications. Precision optical elements require highly polished surfaces or exacting surface figures and qualities. The surfaces demand fabrication in proper geometric relationship to each other; and, where the elements are to be used in transmission applications, they will be prepared from a material of controlled, uniform, and isotropic refractive index.
Numerous methods and materials may be used to fabricate complex, precision optical elements. Because a great majority of conventional machining processes for the manufacture of optical components are unsuited for producing very small features, components having surface features or dimensions of 500 microns or smaller typically can be fabricated only through a few methods of limited applicability. Fabrication of microstructured surfaces using polymers have leveraged off of processes developed by the semiconductor industry for making integrated circuits. Using photolithography and ion etching techniques, some have created submillimeter surface features. These methods, however, are not conducive to large-scale manufacturing. The process time needed to etch a microstructure is proportionally dependent on the required total depth of the microstructure. Moreover, the methods are not only expensive, but can produce only a limited range of feature types. Also, etching processes can create rough surfaces. A smooth concave or convex profile or true prismatic profiles cannot be readily achieved using either of the two aforementioned techniques.
Molding or hot embossing of plastics or glass materials, on the other hand, can form submillimeter-sized features. Plastics can conform to molds and reproduce faithfully intricate designs or fine microstructures. Unfortunately for many telecommunication applications, plastic materials are not ideal since they suffer from several shortcomings. Plastic materials are often not sufficiently robust to withstand, over time, environmental degradation. First, they exhibit large coefficients of thermal expansion, and limited mechanical properties. Plastic optical devices often cannot long withstand humidity or high temperatures. Both the volume and refractive indices of plastics can vary substantially with changes in temperature, thereby limiting the temperature range over which they may be useful. Plastics cannot transmit high-power light, due to internal heating of the material. Thus, well before a plastic component actually melts, its surface features will degrade and its index of refraction may change. Either change is unacceptable in an optical context. Furthermore, since plastics for optical applications are available in a limited range of dispersion and refractive index, plastics can provide only a restricted transmission range. Hence, their usefulness even within the restricted bandwidth is limited by the tendency to accumulate internal stresses, a condition that results in distortion of transmitted light during use. In addition, many plastics can scratch easily and are prone to yellowing or developing haze and birefringence. Application of abrasive-resistant and anti-reflective coatings, unfortunately, still has not fully solved these flaws. Finally, many chemical and environmental agents degrade plastics, which makes them difficult to clean effectively.
In comparison, glass possesses properties that make it a better class of optical material over plastics. Glass normally does not suffer from the material shortcomings of plastics, and it can better withstand detrimental environmental or operational conditions. Hence, glass is a more preferred material. Glass optical components represent a different class of devices than those made from plastics and the molding processes used are more stringent.
Precision optical elements of glass are customarily produced by one of two complex, multi-step processes. In the first, a glass batch is melted at high temperatures and the melt is formed into a glass body or gob having a controlled and homogeneous refractive index. Thereafter, the glass body may be reformed using repressing techniques to yield a shape approximating the desired final article. The surface quality and finish of the body at this stage of production, however, are not adequate for image forming optics. The rough article is fine annealed to develop the proper refractive index and the surface features improved by conventional grinding and polishing practices. In the second method, the glass melt is formed into a bulk body, which is immediately fine annealed, cut and ground into articles of the desired configuration.
Both of these methods have their limitations. On one hand, grinding and polishing are restricted to producing relatively simple shapes, such, such as flats, spheres, and parabolas. Other shapes and general aspheric surfaces are difficult to grind and complicated to polish. On another hand, conventional techniques for hot pressing of glass do not provide the exacting surface features and qualities, which are required for clear image forming or transmission applications. The presence of chill wrinkles in the surface and surface figure deviations constitute chronic afflictions.
The molding of glass traditionally has presented a number of other problems. Generally, to mold glass one must use high temperatures, typically greater than about 700° C. or 800° C., so as to make the glass conform or flow into a requisite profile as defined by a mold. First, at such relatively high temperatures, glass becomes highly chemically reactive. Due to this reactivity of glass, highly refractory molds with inert contact surfaces are required. Some materials used to fabricate molds include silicon carbide, silicon nitride or other ceramic materials, or intermetallic materials, such as iron aluminides, or hard materials, such as tungsten. In many cases, however, such materials do not present sufficient surface smoothness or optical quality for making satisfactory optical surface finishes. Precision optical elements require highly polished surfaces of exacting microstructure and quality. Metal molds can deform and re-crystallize at high temperatures, which can adversely affect the surface and optical qualities of the article being molded. This means additional costs to repair and maintain the molds and higher defects in the product. Second, also due to the reactivity of the glass at high temperatures, often the molding need to be done in an inert atmosphere, which complicates the process. Third, the potential for air or gas bubbles to be entrapped in the molded articles is another drawback of high-temperature molding. If captured within the glass, gas bubbles tend to degrade the optical properties of the article. The bubbles distort images and generally disrupt optical transmission. Fourth, even at high temperatures, hot-glass molding cannot create efficiently on the surface intricate, high-frequency, submillimeter microstructures, such as those required for diffraction gratings.
In the past, workers in the field of molding technology have endeavored to develop several techniques for the manufacture of optical elements. These techniques, however, have yet to satisfactorily overcome the deficiencies of glass molding. Hence, a new method or an improvement of existing technology is needed to for the manufacture of precision optical elements with deep or fine microstructures, such for diffraction gratings or microlenses. The method should be cost-efficient, expedient and enable high-volume, mass production of fine-figured microstructures in multiple, identical glass optical elements. The present invention can satisfy these needs.